If possible, you ought to allow for 3-5h of transit time in Johannesburg, so that you've ample time for you to capture your connecting flight. Transiting in Johannesburg is quite easy; simply Keep to the signs for ‘Global Transit’. The flight from Johannesburg to Harare can take about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/